You ill never get an itinerary with an invalid connection time in conjunction with any fare sold, however if you stick to flexible fares, there is nothing to stop you getting earlier trains if you can make an earlier connection.Agree and I think you only pay one admin fee on flexible tickets. However it still does not solve the problem of minimum connection times. eg. tutbury and hatton to preston where I want to go via crewe. The train gets into crewe at 3 minutes past the hour and the train from crewe goes at 9 mins past the hour. I have not found a search engine yet that provides an itinerary for a 6 minute wait , all I get is an itinerary for a 66 minute wait!
If you want to plan a journey with negative connections (and not be issued any fares), you can do so at www.fastjp.com, in Advanced options, specify the location where you want to reduce the interchange time and input a negative value. This is taken from the minimum interchange time (e.g. if you specifed change at: CRE -5 mins, this would deduct 5 from the 10 minute connectional allowance applicable to Crewe and provide interchanges that are 5 minutes or longer)